The present invention relates, as indicated, to an airflow sensor assembly, and relates more particularly to a sensor assembly for monitoring the breathing of patients and in which a bead thermistor is employed to sense quickly and accurately differences between ambient temperature and the temperature of expired air, which temperature differences result in resistance changes in the thermistor.
The use of thermistors is widely known for diagnosing sleep apnea. In thermistor sensor systems, a thermistor is placed adjacent each nostril or naris of the nose and also adjacent the mouth so that nasal and oral airflow can be monitored. When breathing is normal, the monitoring circuit to which the thermistors are connected will pick up the resistance change in the thermistors due to the temperature differential between ambient and expired air, and provide a read out indicative of normal breathing. When there is at least a temporary cessation of expiration of air, this condition can be quickly picked up by the thermistors, with the sensor system typically providing both visual and audible alarms.
In the past, the use of thermistors in sensor assemblies has presented certain manufacturing and use problems. One proposed solution is disclosed in U.S. Pat. No. 5,190,048 of the present inventor. There is disclosed therein the use of chip thermistors mounted on a thin laminated film assembly. Although the sensor assembly employing the chip thermistors has proved satisfactory in use, there were certain attendant disadvantages both with respect to the manufacturing process and in the care of the assembly. Due to the dimensions and fragility of the chip thermistor, it was not possible to position the chip thermistor in sealed position between the top and bottom layers of film. As a result, it is necessary to laminate the film layers together so as to accommodate the location of each chip thermistor which protrudes exteriorly of the top layer of the assembly. Then, in order to electrically isolate the thermistors electrically and the adjacent conductors at each thermistor location, it is necessary to apply an inert thermally conductive conformal coating over the thermistors in order to seal the entire area.
The design disclosed in the '048 patent also had certain other disadvantages due to the size of the chip thermistor. In order to reduce the mass around the thermistor mounting, it was desirable to incorporate holes or openings in the laminated film on both sides of the long axis of the chip thermistor. This provides airflow around the thermistor so that it can respond better to temperature changes. In addition, the dimension in the area of each thermistor mount was such that it was possible to occlude the nares of infants and neonates. The use of thermistors of reduced size therefore became very desirable.